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Application of the WEPP Model with Digital Geographic Information GIS/EM4 No 4th International Conference on Integrating GIS and Environmental Modeling (GIS/EM4): Problems, Prospects and Research Needs. Banff, Alberta, Canada, September 2 - 8, 2000. Application of the WEPP model with digital geographic information GIS/EM4 No. 149 Dennis C. Flanagan Chris S. Renschler Thomas A. Cochrane Abstract The Water Erosion Prediction Project (WEPP) is a process-based continuous simulation erosion model that can be applied to hillslope profiles and small watersheds. One limitation to application of WEPP (or other models) to the field or farm scale is the difficulty in determining the watershed structure, which may be composed of multiple channels and profiles (and potentially other features as well). This presentation describes current efforts to link the WEPP model with Geographic Information Systems (GIS) and utilize Digital Elevation Model (DEM) data to generate the necessary topographic inputs for erosion model simulations. Two automated approaches for applying the WEPP model have been developed and compared to manual application of the model. The first approach (named the Hillslope method) uses information from a DEM to delineate the watershed boundary, channel and hillslope locations, and then configure "representative" hillslope slope profiles from the myriad flowpath data. The second approach (named the Flowpath method) also uses DEM information to delineate the watershed boundary, but then runs WEPP model simulations on every flowpath within a watershed. For a set of research watersheds, the automatic Hillslope method performed as well as a manual application of WEPP by an expert user in predictions of runoff and sediment loss. Tests also showed that the Hillslope and Flowpath methods were not significantly different than each other or different from manual model applications in predictions of hillslope erosion. Additional research work ongoing at the National Soil Erosion Research Laboratory is examining the feasibility of using commonly available digital elevation data (for example from on-vehicle Geographical Positioning Systems (GPS)) to provide input for the automated techniques for driving the erosion model. Keywords Soil erosion prediction, process-based model, DEM, WEPP. Introduction The Water Erosion Prediction Project (WEPP) model was developed from 1985-1995, by the United States Departments of Agriculture and Interior, and was publicly released in 1995 for application on cropland, rangeland, forestland, and other managed lands (Flanagan and Nearing 1995). WEPP simulates the important physical processes that result in soil erosion by water. The model contains a climate generator, simulates surface and subsurface hydrology, irrigation, plant growth, residue decomposition, effects of tillage, soil detachment by raindrop impact and flowing water, sediment transport and deposition. At the time of its initial release, basic DOS interface programs were provided to assist users in creating input files, managing groups of simulation runs, and generating and viewing output. Since about 1997, next generation WindowsTM compatible interfaces for WEPP have been under development and work is near completion on a graphical watershed interface. However, even with an excellent graphical interface, anything but relatively simple watersheds can still become too difficult for model users to delineate, because of the potentially large numbers of channel nodes and hillslope profiles required. Approaches are needed that can access and use digital elevation data to delineate watershed boundaries, channel locations and slope, hillslope profile locations, and profile topographic inputs required by the erosion model. Additionally, good quality digital elevation data is lacking in many locations in the world, so another current research area is examining the possibility of using elevation data obtained from on-vehicle Geographical Positioning Systems to provide topographic input data for erosion models. The watershed and hillslope modeling work described here involves linkage of the WEPP model with the ArcViewTM 3.0 GIS (software developed by Environmental Systems Research Institute, Inc.) and TOPAZ (TOpographic PArameteriZation tool, developed by the USDA-Agricultural Research Service). TOPAZ (Garbrecht and Martz 1997) was used to delineate watersheds, locate channels, delineate hillslope profiles, and provide information on flow paths within the profiles. New techniques were developed and evaluated to determine representative slope profile inputs based upon the flow paths. ArcViewTM was used to process and display the erosion model inputs and outputs. Problem statement The major problem addressed by this area of work is "Can automatic techniques be developed and used to delineate a watershed into a reasonable representation of what is physically present, and provide for accurate predictions of runoff and soil loss?" In other words, does a delineated watershed have the proper area, number of hillslope profiles and number of channels? Are the hillslope profiles equivalent to those defined by an expert user, and can they produce model simulation results that are comparable to measured runoff and sediment loss data? Also, if fine resolution DEM data is lacking, are there other commonly available sources of data that can be used to drive the automatic techniques, and what is their impact on model simulation results? Background Integration of WEPP or other erosion prediction models with a GIS can facilitate and possibly improve the application of the technology. Savabi et al. (1995) applied the WEPP model at the Purdue University animal science watershed, using a GIS to obtain some of the physical parameters required by WEPP. However, digital elevation data was not available or used in that study. The primary layer required in a GIS to delineate hillslopes and channels is a topography map. Topography is usually represented in a GIS as a Digital Elevation Model (DEM) or a Triangular Irregular Network (TIN). Most DEMs are grid-based, where each elevation point is represented by a cell of a certain size or resolution. Flow-routing algorithms that determine the steepest descent direction and gradient between cells can be used to delineate watershed boundaries, hillslopes and channels. Moore et al. (1991) and Desmet and Govers (1996) provide descriptions of many current flow-routing algorithms. These types of procedures have often been used to integrate simple erosion equations such as the Universal Soil Loss Equation (USLE) (Wischmeier and Smith 1978) with GIS. The ANSWERS, AGNPS, and SWAT (Srinivasan and Arnold 1994) models also all use flow-routing algorithms and GIS maps to predict erosion or runoff. Thus, watershed analysis using digital geographic information has good potential for parameterization of hillslopes, channels, and representative slope profiles for WEPP model simulations. Methods The work presented in this paper falls into two areas. First, we present information on approaches to automatically delineate watersheds into hillslopes and channels and conduct WEPP model simulations. Second, we describe a recent study to obtain digital elevation data for an agricultural watershed from commonly available GPS technology. Automatic watershed and hillslope delineation In terms of automation of WEPP applications through GIS linkage, two automated approaches were developed and tested against manual applications of the model to a set of six research watersheds which had well-documented soil, management, climatic, topographic, runoff, and soil loss data. The manual application of the model used topographic slope profile inputs to WEPP created by erosion prediction experts, who had for their use field observations, aerial photographs, and detailed contour maps of the sites (Kramer 1993, Liu et al. 1997). WEPP model inputs assume rectangular hillslope areas contributing water and sediment laterally and/or to the top of channel segments (Figure 1). Figure 1. Steps in discretizing a watershed for a WEPP model simulation. The test watersheds used were one from Treynor, Iowa, three from Holly Springs, Mississippi, and two from Watkinsville, Georgia, with watershed area ranging from 0.6 to 29 ha (Table 1). A TIN for the Treynor watershed was obtained by aerial surveys with ground control (90 points per hectare). For the Holly Springs and Watkinsville watersheds, elevation data were obtained from field survey maps with 5 ft contours, which were digitized. A grid-based DEM was then created for each watershed. Main Years of Area Soil type / Location Watershed soil Crops simuation [ha] series texture Monona- Treynor, IA W2 1985-1990 29 Silt loam Corn Ida-Napier Wheat Sandy sorghum, loam / Watkinsville,GA P1 1972-1974 2.7 Cecil barley, sandy soybean, clay loam clover Sandy Corn, Watkinsville, loam / P2 1973-1975 1.29 Cecil bermuda GA sandy grass clay loam Holly Springs, Soybean, WC1 1970-1977 1.57 Grenada Silt loam MS meadow Corn, Holly Springs, Silt WC2 1970-1977 0.59 Grenada wheat, MS Loam soybean Corn, Holly Springs, WC3 1970-1977 0.65 Grenada Silt loam wheat, MS soybean Table 1. Research watersheds used in testing automatic procedures. Hillslope method The first automated approach (named the Hillslope method) uses information from a DEM to delineate the watershed boundary, channel and hillslope locations, and then configures a single "representative" slope profile from the flow path information. TOPAZ (Garbrecht and Martz 1997) is used for extraction of flow routing features in a watershed, and was chosen for its ability to
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